Process for determining the particle size distribution of an aerosol and apparatus for carrying out such a process

ABSTRACT

The measurement of the particle size distribution of an aerosol is of crucial importance in pharmaceutical development. A prerequisite for this is a valid method, e.g. the cascade impactor method described in the pharmacopoeias. It requires measurement of the active substance concentration in order to determine the particle size distribution. The invention provides a process which makes it possible to measure the size of aerosol droplets containing pharmaceutical substances which are unstable in the event of changes in the pH.

BACKGROUND OF THE INVENTION

[0001] The invention relates to processes for determining the sizedistribution of the particles contained in an aerosol, especially theparticles of a pharmaceutical formulation.

[0002] The invention also relates to an apparatus for carrying out sucha process.

[0003] Within the scope of the invention the term “pharmaceuticalsubstance” refers to the active ingredient of a medicament which isusually also known as a drug or active substance.

[0004] The term “pharmaceutical formulation” is to be interpretedbroadly, to cover formulations in the form of solutions, suspensions andpowders, in particular. In a solution formulation the pharmaceuticalsubstance is dissolved in a solvent, whereas in a suspension or powderformulation the pharmaceutical substance is present in solid form.Whereas in a suspension formulation it is mixed with a suspension agentand the pharmaceutical substance is contained in this suspension agentin the form of suspended particles, a powder formulation does not haveany solvent or suspension agent in this sense but is present to someextent in pure form, as a pure powder.

[0005] A solution formulation is prepared and metered using an atomiseror nebuliser, preferably a nebuliser, in which a quantity of less than100 ml, preferably less than 50 ml, preferably less than 20 ml of theformulation is prepared.

[0006] An apparatus of this kind for propellant-free nebulising of ametered amount of the above-mentioned pharmaceutical formulations isdescribed in detail, for example, in International Patent Application WO91/14468 “Atomizing Device and Method” and also in WO 97/12687, FIGS. 6aand 6 b. In a nebuliser of this kind a pharmaceutical formulation isconverted into an aerosol by the use of high pressure, up to 500 bar,the particles introduced having a diameter of less than 100 μm,preferably less than 20 μm.

[0007] Apart from this device, other inhalers known from the prior artmay also be used in the process according to the invention, such as theMDI (metered dose inhaler) or powder inhalers such as the one known bythe trademark HandiHaler®, for example.

[0008] In nebulisers of this kind the formulations are stored in areservoir and for this reason the formulations used must be sufficientlystable when stored.

[0009] It is essential in the pharmaceutical industry to measure theparticle size distributions of aerosols in order to assess thecharacteristics of deposition in the lungs and bronchial region, as willbe shown hereinafter.

[0010] In a number of applications, particularly in the case of diseasesof the lungs and bronchial region, the pharmaceutical substance isprovided in the form of an inhalable medicament. The pharmaceuticalformulation is atomised to form an aerosol. The aerosol thus producedcan then be transported in a carrier medium, e.g. air.

[0011] For example, when an asthma spray is used, a pharmaceuticalformulation stored in an atomiser is finely atomised through a nozzle,by brief actuation, and introduced into the ambient air breathed in bythe patient, this ambient air acting as the carrier medium. The airenriched with the pharmaceutical formulation forms an aerosol, which isinhaled.

[0012] Inhalable preparations demand a certain form for the medicament.As a rule, micronised pharmaceuticals or active substances in solid formare used. However, in theory, the drug may be present in liquid or solidform, e.g. as a powder, while solid particles do not dissolve in thesolvent in the traditional sense or are present in pure form.

[0013] To ensure that the pharmaceutical substance is capable of beinginhaled, stringent demands are made of the particle size, particle sizedistribution, morphology, stability and flow characteristics.

[0014] As a rule, not all the inhaled dose of the pharmaceuticalsubstance reaches the lungs but only part of this dose. The amount ofthe composition which actually enters the lungs is critically influencedby the particle size. For this reason, particles with a diameter of lessthan 20 μm, preferably less than 5 μm and greater than 0.3 μm arepreferred.

[0015] The diameter of the particle should fall within the rangespecified and should additionally have the narrowest possible sizedistribution. Larger particles are deposited too early, in the upperrespiratory tract, when breathed in, whereas smaller particles are notdeposited in the lungs and are breathed out again.

[0016] For example, when an asthma spray is used, a pharmaceuticalformulation stored in an atomiser is finely atomised through a nozzle,by brief actuation, and introduced into the ambient air breathed in bythe patient, this ambient air acting as the carrier medium. The airenriched with the pharmaceutical formulation forms an aerosol, which isinhaled.

[0017] To ensure that the pharmaceutical substance is capable of beinginhaled, stringent demands are made of the particle size, particle sizedistribution, morphology, stability and flow characteristics.

[0018] As a rule, not all the inhaled dose of the pharmaceuticalsubstance reaches the lungs but only part of this dose. The amount ofthe composition which actually enters the lungs is critically influencedby the particle size. For this reason, particles with a diameter of lessthan 20 μm, preferably less than 5 μm and greater than 0.3 μm arepreferred.

[0019] The diameter of the particle should fall within the rangespecified and should additionally have the narrowest possible sizedistribution. Larger particles are deposited too early, in the upperrespiratory tract, when breathed in, whereas smaller particles are notdeposited in the lungs and are breathed out again.

[0020] By the particle diameter within the scope of the presentinvention is meant the aerodynamic particle diameter, which is definedas the equivalent diameter of a sphere with a density of 1 g/cm³ whichhas the same sedimentation speed in air as the particle underinvestigation.

[0021] Against this background it is easily understandable that thepharmaceutical industry has a need for a process which can be used todetermine the particle size distribution of aerosols.

[0022] However, the legislators, and particularly the healthauthorities, also demand accurate knowledge of the dose that is actuallyadministered, i.e. the proportion of the total dose inhaled which isdeposited in the lungs and bronchial region.

[0023] Moreover, apart from the absolute quantity administered, the sizedistribution affects the bioavailability of the pharmaceutical substancein that, although the absolute amounts are the same, a large number ofsmall particles have a different bioavailability from a small number oflarge particles.

[0024] According to the prior art, three conventional methods are usedto determine the particle size distribution.

[0025] A first, widely used method of determining particle sizedistribution is the so-called impaction method using the Andersencascade impactor. The cascade impactor is a standardised apparatus forcarrying out a standardised measuring process, the so-called impactionmethod; both the process and the apparatus are described in detail indrugs manuals (cf. also European Pharmacopoeia, 3^(rd) Edition,Supplement 2001, 2.9.18 Preparation for inhalation).

[0026]FIG. 1 shows the Andersen cascade impactor in diagrammatic sideview and partially in section (loc. cit. page 122). The cascade impactor(1) is acted upon by the aerosol which is under investigation throughthe inlet opening (3) of a right-angled inlet tube (2).

[0027] The inlet (2) is a standardised component (loc. cit. page 120)which is also known as a USP throat and simulates theoropharyngeal-cervical cavity in humans. To illustrate the USP throat,FIG. 2.9.17-7 (induction port) is reproduced in FIGS. 2a to 2 d.

[0028]FIG. 2d shows the USP throat in perspective view, while FIGS. 2ato 2 c serve to illustrate the dimensions envisaged. FIGS. 2a to 2 d areintended to give an overall impression of the USP throat and show thatit is a component with an extremely detailed specification, leaving noleeway for the manufacturer or user.

[0029] As with the pharmaceutical formulation administered to thepatient with the ambient air breathed in, with which it forms anaerosol, is passed through the oral and pharyngeal cavities into thewindpipe and from there is passed into the lungs to the bronchi, in theAndersen cascade impactor (1) as well the aerosol is conveyed along acurved flow path through the non-linear USP throat (2) to the actualsample collector (5).

[0030] In accordance with human anatomy, the aerosol flow through theentry opening is conveyed into a first section (2 a) of the USP throat(2) and then into a second section (2 b) which is connected to the firstsection (2 a) and arranged substantially perpendicular thereto.

[0031] The particles of the aerosol are subjected to radially outwardlydirected centrifugal forces on account of the nonlinear direction offlow and the resulting curved flow path. If the mass of the aerosolparticles exceeds a certain size, these particles can no longer followthe deflected flow but are deposited on the walls of the USP throat (2).

[0032]FIG. 1 shows the flight path (12) of a particle which cannotfollow the direction of flow and hits or is deposited on the inner wallof the second section (2 b) of the USP throat (2).

[0033] This is in principle the first stage of the Andersen cascadeimpactor which simulates the deflection of the aerosol breathed in bythe patient in the pharyngeal cavity and the resulting deposition ofpharmaceutical formulation in the pharyngeal cavity.

[0034] The USP throat (2) is connected to the actual sample collector(5) via a connecting member (4), which is also standardised (loc. cit.,page 123). The aerosol flow expands in the connecting member (4) and isguided towards the first stage or cascade (6 ₁) of the cascade impactor(1)

[0035] The cascade impactor (1) is a substantially cylindrical containerof modular construction through which the aerosol fed in travels fromtop to bottom, passing through a number of stages, the so-calledcascades, while the aerosol particles contained in the carrier mediumare deposited in a sequence from coarse to fine or from heavy to light.

[0036] Each stage or cascade (6 ₁, 6 ₂, 6 ₃, 6 ₄, 6 ₅, 6 ₆, 6 ₇, 6 ₈, 6₉) comprises a plurality of impactor nozzles (7 ₁, 7 ₂, 7 ₃, 7 ₄, 7 ₅, 7₆, 7 ₈). An impactor nozzle (7) of this kind is shown diagrammaticallyin side view and in section in FIG. 3.

[0037] The aerosol which acts on the nozzle (7) is deliberatelyaccelerated in the inlet aperture (8) of the nozzle (7) by a definedconstriction of the cross section of the nozzle entrance and thendeflected by means of an impactor plate (11). As in the deflection offlow in the USP throat (2), here too the curved path of movement and thecentrifugal forces acting on the particles as a result cause particlesof a certain mass to be deposited.

[0038]FIG. 3 shows the flow lines (10 ₁, 10 ₂) of the aerosol flow,which the lighter particles essentially follow without colliding withthe impactor plate (11). FIG. 3 also shows the flight path (12) of aparticle striking the impactor plate (11) because of its excessivelygreat mass.

[0039] The nozzle (7) acts to some extent as a filter for filtering outparticles exceeding a given mass from the aerosol flow and depositingthem on the impactor plate (11). Because of the fact that it is astandardised apparatus and a standardised process, accurate informationis available as to the conditions in the region of the nozzle. For eachcascade the precise mass m_(ab) of the particles deposited on theimpactor plate (11) here is known.

[0040] After passing through the first stage or cascade (6 ₁) and afterthe first depositing of heavy particles, the aerosol passes througheight more cascades (6 ₂ to 6 ₉) as shown in FIG. 1, while the geometryof the impactor nozzles (7) varies or becomes finer from stage to stageand allows finer and finer, i.e. lighter, particles to be filtered out.

[0041] The aerosol particles deposited in a certain stage thus have aspecific mass which is within a very narrow window bounded by an upperand lower limit.

[0042] As the last stage (not shown in FIG. 1) a filter may be providedwhich collects all the particles that have not previously been depositedand thus, together with the impactor plates (11), makes it possible todetermine the absolute total mass of the pharmaceutical formulation fedinto the impactor.

[0043] After the aerosol has passed through the impactor, the impactorplates (11) of each cascade are removed and subjected to extensiveanalysis. The main priority is to determine the particle sizedistribution. Theoretically, first of all the total mass ofpharmaceutical formulation impacted or deposited on each impactor platecould be determined. By knowing the mass m_(ab) of the particlesdeposited in each stage the number of particles deposited in eachcascade can be calculated.

[0044] In practice however, it has been found that the measurements thusobtained are not reproducible within narrow limits. Tests have shownthat some of the pharmaceutical formulation evaporates during themeasuring process.

[0045] Because of its unsaturated state the carrier medium can absorbadditional liquid, and therefore liquid may be, and generally is, givenoff from the particles to the carrier medium by evaporation.

[0046] The evaporation of the particle droplets leads to a change in theparticle mass of each individual particle and hence to a reduction inthe particle diameter and consequently to a measurement which isfalsified by evaporation.

[0047] The fact that the effects of evaporation may be significant isdemonstrated in FIG. 4, which shows the lifespan of a drop of waterdepending on the initial droplet diameter for various relativehumidities (0%, 50%, 100%) at 20° C. (William C. Hinds “AerosolTechnology—Properties, Behavior, and Measurement of airborne Particles”,page 270, ISBN 0-471-08726-2).

[0048] Small particles have a short lifespan and may evaporate duringmeasurement, so that they are no longer taken into consideration withinthe scope of the particle size distribution.

[0049] For the reasons stated it is not sufficient just to determine thetotal mass of the pharmaceutical formulation deposited on each impactorplate. Moreover, the pharmaceutical formulation of each impactor platehas to be subjected to analysis to determine the concentrations of thecontents. Partial evaporation of the solvent or suspension agent causesconcentration of the pharmaceutical substance and the other ingredientsof the aerosol droplet.

[0050] On the basis of the degree of concentration of the pharmaceuticalsubstance conclusions are drawn as to the fraction of the particle whichis evaporated. Taking this evaporated fraction into consideration actsas a corrective when determining the particle size distribution andleads to a different total mass for the pharmaceutical formulationdeposited on each impactor plate.

[0051] The pharmaceutical formulation deposited on the impactor plates(11) may, for example, be analysed for its composition by the HPLCmethod.

[0052] Further tests have shown that in spite of taking account of theevaporation as described above, the results still vary from onemeasurement to the next and it is desirable to increase thereproducibility still further. Other tests have shown that simplyallowing for the mass of the evaporated fraction of the aerosolparticles is not enough.

[0053] If a liquid pharmaceutical formulation contains acids or bases toadjust the pH, the evaporation that occurs during measurement leads to araising or lowering of the pH. Consequently, in pH-sensitive activesubstances, decompositions occur, with the result that the analysis ofthe concentration of the active substance in the fractions deposited nolonger corresponds to the concentration which was actually present inthe original aerosol droplet. This procedure thus comes up against itslimitations and can only be used to a limited extent as a corrective forthe evaporation effect. A pharmaceutical solution or suspension might,for example, contain an acid X as excipient, in addition to thepharmaceutical substance A and water as the solvent. If some of thesolvent water is evaporated, the pH is lowered and the particle becomesincreasingly acidic, triggering breakdown of the pharmaceuticalsubstance. The same is true if the pharmaceutical formulation contains abase.

[0054] The analysis, i.e. the evaluation of the measurements made withthe cascade impactor, is extraordinarily time-consuming andlabour-intensive. The entire apparatus is taken to pieces in order togain access to the multiplicity of impactor plates (11). Each impactorplate is weighed and analysed. Thus, as a rule, only a few measurementscan be done per day and there is a considerable time span between theactual measuring and the results of the measurements becoming available.

[0055] Another process for determining the particle size distribution ofan aerosol, which is far less time-consuming and labour-intensive thanthe impactor method, is the so-called laser diffraction method. Unlikethe impactor method the laser diffraction method does not require anycomplex analysis and therefore makes it possible to work considerablyfaster and to obtain the results of the measurements much more quickly.

[0056] DIN-ISO 13320-1 (First Edition Nov. 1, 1999) describes laserdiffraction processes. In them, parallel light is transmittedperpendicular to an aerosol flow using a laser. The particles containedin the aerosol flow obstruct the laser beams, with the result that thelight beams are diffracted on the particles. The scattered lightemerging at the opposite side of the incident laser beam, which isgenerated by the diffraction of the laser beams on the particles,produces a circular interference pattern with concentric rings and isfed to a detector, usually a semiconductor detector. The usual methodsof evaluating this interference pattern are Mie scattering and theFraunhofer method.

[0057] Whereas the aerosol flow to be investigated is generally fedcontinuously to the cascade impactor through a USP throat, the particlesize distribution of an aerosol is determined by the laser diffractionmethod according to the prior art on the free-flowing aerosol, i.e.usually on a one-off, conical, inhomogeneous and thereforenon-reproducible metered stream.

[0058] However, a disadvantage of the laser diffraction method, as withthe impactor method, is the fact that some of the aerosol particles areevaporated and thus the measurements are falsified. Although measurementof the aerosol by the laser diffraction method can be carried out muchfaster than the impactor method, in which the aerosol flow has to travelalong a long flow path, the evaporation effect also plays a part in thelaser diffraction method.

[0059] Precisely in the case of aerosols in which the particles are inthe form of drops of liquid there is a danger of at least partialevaporation of the liquid aerosol particles, which means that theeffects described are particularly significant in formulations in theform of solutions and suspensions.

[0060] The functional correlation shown in FIG. 4 and the need to takeit into account were verified experimentally.

[0061] Experiments with the cascade impactor at various relativehumidities have shown that measurements of the particle sizedistribution of an aerosol should most sensibly be carried out at highrelative humidities, as the evaporation effect crucially influences themeasurements if the humidity of the air is too low and finally highhumidity levels also correspond to the actual conditions in the humanoropharyngeal-cervical cavity. It should be taken into considerationthat the processes described are to be used to determine thecharacteristics of deposition in the lungs and bronchial region, and forthis reason every attempt should be made to simulate the conditionsprevailing therein, i.e. pressure, temperature and humidity.

[0062] A third method of measuring aerosols is the scattered lightmethod. Such a method is described by Dr. -Ing. H. Umhauer in VDIBerichte 232 (1975), pages 101ff. “Ermittlung vonPartikelgröβenverteilung in Aerosolströmungen hoher Konzentration nitHilfe der Streulichtmethode”.

[0063] This method is suitable for measuring very fine particles with adetection limit which may be well within the submicroscopic range. Themeasuring process is set up as a counting process which detects theparticle size and counts the individual particles so that it is possibleto make pronouncements as to both the quality and quantity of theparticles.

[0064] A disadvantage of the scattered light method is that a smallmeasuring volume has to be defined, e.g. with sides 100 :m long, becauseonly one particle may ever stop in the measuring volume if clearscattered signals are to be obtained. The apparatus and the calibrationthereof are correspondingly complex. In practice the scattered lightmethod has proved to be inferior to the laser diffraction method as itsresults are less reliable.

[0065] Moreover, the scattered light method, like the laser diffractionmethod, suffers from the evaporation effect with the associateddisadvantages.

[0066] As in the conventional laser diffraction process, the scatteredlight method is also carried out on a free flow of aerosol.

[0067] To sum up, it can be said of the prior art that the cascadeimpactor is highly time-consuming and costly because of its complicatedanalysis, whereas the scattered light method and also the laserdiffraction process, while offering comparatively fast processes, sufferfrom the evaporation effect, like the cascade impactor.

[0068] Against this background the problem of the present invention isto provide a process for determining the particle size distribution ofan aerosol which counteracts the disadvantages of the prior artdescribed, and which minimises the influence of the evaporation of theaerosol particles and increases the reproducibility of the measurements.

[0069] A further aim of the process according to the invention is tomeasure liquid pharmaceutical aerosols for their particle sizes whichcontain an active substance which responds to changes in pH by breakingdown.

[0070] Another secondary aim of the invention is to provide an apparatusfor carrying out such a process.

BRIEF SUMMARY OF THE INVENTION

[0071] The present invention provides a process to determine theparticle size distribution of an aerosol, especially an aerosol of apharmaceutical formulation. The process of the present inventioncomprises the following steps:

[0072] preparing a carrier medium which is saturated with a conditioningagent according to a given degree of saturation,

[0073] mixing the conditioned carrier medium with the pharmaceuticalformulation to produce a conditioned aerosol, and

[0074] introducing the conditioned aerosol into at least one measuringcell and measuring the conditioned aerosol in order to determine theparticle size distribution of the aerosol.

BRIEF DESCRIPTION OF THE DRAWINGS

[0075]FIG. 1 is a side view of an Anderson impactor with an USP throat.

[0076]FIGS. 2a to 2 d show an USP throat with standardised geometricmeasurements.

[0077]FIG. 3 shows an impactor nozzle.

[0078]FIG. 4 is a chart showing dependency of droplet size on lifespanof droplet for three(3) different degrees of saturation with water.

[0079]FIG. 5 is a chart comparing properties of a conventional USPthroat with one modified with an angled measuring cell according to thepresent invention.

[0080]FIGS. 6a to 6 b show side views of an angled measuring cell.

[0081]FIG. 7 shows an apparatus for measuring the particle sizedistribution of an aerosol according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0082] Using the process according to the invention, the carrier mediuminto which the aerosol is introduced is enriched with a conditioningagent according to a given degree of saturation. This measurecounteracts any evaporation of the particles of a formulation in theform of a solution or suspension and all the other disadvantagesconnected with evaporation, particularly in substances which aresensitive to changes in pH.

[0083] In addition, the enrichment of the carrier medium with aconditioning agent mimics the actual conditions—particularly the highlevel of humidity—in the human oropharyngeal-cervical cavity, therebycreating measuring conditions which are as realistic as possible.Additionally the temperature and pressure can be adjusted as required.

[0084] The conditioning agent thus has two functions, while theestablishment of a relative humidity, for basically all pharmaceuticalformulations, which corresponds to the actual conditions ensures thatthe particle size distribution determined for the aerosol is asrealistic as possible, thereby increasing the reliability of themeasurement for evaluating the deposition characteristics in the lungsand bronchial region.

[0085] In the case of solution or suspension formulations the degree ofsaturation of the carrier medium is preferably more than 80%, morepreferably more than 90%, most preferably more than 95%. Ideally it is100%.

[0086] The carrier medium conditioned in this way is mixed with thepharmaceutical formulation under investigation in accordance with theprocess according to the invention to produce a conditioned aerosol.Atomisers or nebulisers as described in the introduction to thespecification may be used for this purpose in the case of pharmaceuticalsolutions, for example.

[0087] Advantageously, once again an attempt is made to simulate realityas accurately as possible, for which reason the atomiser is preferablyarranged in a mixing chamber in which the aerosol produced by theatomiser is mixed with the carrier material (air or some other gas) inorder to direct the mixture in a defined flow from there through themeasuring device. One advantage of the aerosol being guided in this wayis that this approximates very closely to the conditions that prevailwhen an aerosol is being breathed in by a patient.

[0088] On the other hand, the carrier flow avoids any impact on thewindow of the measuring cell(s) in the case of liquid aerosols, which isan advantage particularly in the laser diffraction method. To producethis effect it is not necessary for the carrier material to beconditioned, i.e. saturated with water.

[0089] The carrier material may, for example, flow through slots in themouthpiece of the inhaler into the interior of the mouthpiece, where itis mixed with the cloud of aerosol produced by the inhaler andintroduced therein.

[0090] The conditioned aerosol thus generated is then fed into at leastone measuring cell and measured in order to determine the particle sizedistribution of the aerosol.

[0091] In an advantageous process, the measurement of the conditionedaerosol is carried out by the laser diffraction method. The laserdiffraction method is a fast method which gives rapid access to themeasurements.

[0092] In an advantageous process, the measurement of the conditionedaerosol is carried out by the scattered light method. The scatteredlight method gives not only a qualitative result—the particle size—butalso a quantitative result, as the particles are not only measured fortheir size but are also counted.

[0093] In an advantageous process the measurement of the conditionedaerosol is carried out by the impactor method. As the impactor method isthe one which is most widespread—inter alia because it is recommendedand accurately described in the pharmacopoeias—and has been used for agreat many measurements, this process offers a large fund of comparativemeasurements. Thus, when new pharmaceutical formulations and/or newmetering devices, particularly atomisers, are being tested it may besensible to compare them with conventional pharmaceutical formulationsor metering devices.

[0094] In an advantageous process the measurement of the conditionedaerosol is carried out by the laser diffraction process and thescattered light method. Using the two methods together combines theadvantages of both methods, namely the accuracy of the laser diffractionprocess in qualitative terms with the scattered light method as aparticle counting method and hence the possibility of makingquantitative pronouncements.

[0095] Favourable methods are characterised in that the conditionedaerosol is also measured by the impactor method. In this alternativeembodiment of the process the impactor method additionally used servesprimarily to verify the measurements obtained by the process accordingto the invention. Tests have shown that the measurements obtained withthe two methods largely agree.

[0096] In the process according to the invention, the particle sizedistribution of the aerosol may optionally be carried out either by thelaser diffraction method or by the scattered light method or using bothmethods, i.e. the laser diffraction method and the scattered lightmethod, while if both methods are used they are carried out together inone measuring cell or a separate measuring cell is provided for eachprocess, so that both processes have their own measuring cell to suittheir own particular requirements.

[0097] Experimental tests have shown that the process according to theinvention, particularly the laser diffraction method, in a conditionedaerosol flow agrees closely with the measurements obtained using theAndersen cascade impactor.

[0098] In an advantageous embodiment of the process in which a solutionformulation, i.e. a solution of pharmaceutical substance, is used, thesolvent in which the pharmaceutical substance is dissolved is used asthe conditioning agent. In order to prevent evaporation of the solventcontained in the aerosol particles, the solvent whose evaporation is tobe inhibited is advantageously added to the carrier medium as aconditioning agent. The addition of the solvent as a conditioning agentlowers the vapour pressure of the solvent in the carrier medium, therebymaking it more difficult for the liquid aerosol particles to evaporate,or reducing the amount of evaporation.

[0099] In preferred variants of the process in which a suspensionformulation, i.e. a suspension of pharmaceutical substance, is used, thesuspension agent containing the pharmaceutical substance in the form ofsuspended particles is used as the conditioning agent. The reasons forthis are similar to those mentioned for the solution formulation. Inorder to prevent or reduce the evaporation of the suspension agentcontained in the aerosol particles, the suspension agent is preferablyadded as the conditioning agent to the carrier medium, as a result ofwhich the vapour pressure of the suspension agent in the carrier mediumis lowered and hence the tendency to evaporation is counteracted.

[0100] According to favourable variants, if a propellant-free dissolvedor suspended formulation is used, the solvent or suspension agent isused as the conditioning agent. Water, water/alcohol mixtures or alcoholare preferred. The preferred alcohol is ethanol. Water is mostpreferred.

[0101] Because the powder formulation, unlike the solution andsuspension formulations, does not have any solvent or suspension agent,water is preferably added as conditioning agent to the carrier medium.

[0102] This comes very close to the actual conditions because the moistair breathed in by the user, which acts as a carrier medium for thepowder, can be satisfactorily simulated or reproduced experimentally byair saturated with water as conditioning agent.

[0103] In contrast to the other two pharmaceutical formulations, in apowder formulation a preferred process is one wherein the carrier mediumdoes not exceed a saturation level of 75%, as the pharmaceuticalsubstance present in the form of a fine powder tends to form largerparticles and particularly to cake together at higher saturation levels.

[0104] Also advantageous are variants of the process in which air isused as the carrier medium. One reason for this is that again the use ofair as a carrier medium corresponds to the actual conditions and thusprovides a good simulation in the experiment.

[0105] In advantageous processes a USP throat, through which the aerosolflow passes, is used within the scope of the measurement. This simulatesthe flow through the oropharyngeal-cervical cavity.

[0106] In advantageous processes the measurement of the particle sizedistribution of the aerosol by the laser diffraction process and/or thescattered light method is carried out in at least one measuring cellwhich is separated from the surrounding atmosphere. To maintain theconditioning and stabilise the conditioned aerosol, i.e. to separate itfrom the surrounding atmosphere, a suitable prepared measuring cell isadvisable.

[0107] Preferred embodiments of the process are those wherein themeasurement of the particle size distribution of the aerosol by thelaser diffraction method and/or the scattered light method is carriedout in at least one measuring cell integrated in the USP throat. The USPthroat is a test device recognised by the FDA (Food and Drug Agency).

[0108] The or each measuring cell may be arranged both in the firstsection of the USP throat and also in the second section of the USPthroat. Moreover, when two measuring cells are provided, there is thepossibility of arranging one measuring cell in the first section and thesecond measuring cell in the second section of the USP throat, whileonce again it is also possible to arrange both measuring cells in thefirst or second section of the USP throat.

[0109] This or each measuring cell is in fact shown in FIGS. 2a to 2 dand integrated in the USP throat described in detail hereinbefore.

[0110] It is thus possible to carry out the laser diffraction processand/or the scattered light method on a defined, well known aerosol flow.The measurement of the particle size distribution on a free,inhomogeneous and non-reproducible aerosol cloud, which is regarded asdisadvantageous, is thus replaced by a defined process usingstandardised equipment.

[0111] Surprisingly, tests have shown that by suitably modifying the USPthroat in order to integrate at least one measuring cell the advantagesof the laser diffraction process and/or the scattered light method canbe combined with the advantages of the impactor method using the USPthroat, while the experts will be astounded to discover that themodification, i.e. the structural alteration of the USP throat, has noeffect on the measurements obtained for the particle size distribution.

[0112] This has been verified in numerous experiments, in which aconventional cascade impactor was fitted with the modified USP throatand comparative measurements were taken which were compared with themeasurements using the conventional USP throat. The results of thecomparative measurements are shown in FIG. 5, in which the distributionsum Q₃ is shown as a function of the particle diameter. The measurementsmarked with a small square represent the results of a cascade impactorfitted with an original throat, whereas the measurements marked with asmall circle are the measurements obtained with a cascade impactorfitted with the modified USP throat according to the invention. Verygood correspondence is obtained, so that it may reasonably be concludedthat the modification of the USP throat does not substantially alter thedata obtained with the original throat.

[0113] Thus, the process according to the invention which provides forthe arrangement of at least one measuring cell in the USP throat issuitable for replacing the conventional process using cascade impactors,which do not produce rapid measurements on account of the complicatedanalysis required.

[0114] An advantage of this variant of the process is that the USPthroat is an exactly standardised component which simulates theoropharyngeal-cervical cavity in humans, and ensures a precisely definedflow of aerosol.

[0115] According to favourable embodiments of the process, the meteredaerosol is fed into a separator in order to separate off the particles.This is done in order to determine the total absolute particle mass ofthe particles or pharmaceutical formulation delivered with the aerosol.A filter may be used as the separator.

[0116] This is particularly favourable in conjunction with the variantof the process according to the invention in which the laser diffractionprocess is not carried out together with the scattered light method butis used on its own. In this case, in fact, there is a need for anapparatus which will provide quantitative information on the particlemass delivered.

[0117] Whereas the scattered light method provides both qualitative andquantitative results, i.e. gives information on both the sizedistribution and on the particle masses, the laser diffraction processcan only provide information as to the size distribution and not on thequantity of aerosol particles.

[0118] Processes wherein a laser is used as the light source for thelaser diffraction process or the scattered light method are favourable.An advantage of the use of a laser is that it emits parallel light.

[0119] The problem of the equipment required is solved by an apparatushaving:

[0120] a conditioning device for saturating a carrier medium with aconditioning agent according to a given saturation level,

[0121] a metering device for metering and preparing a pharmaceuticalformulation,

[0122] a mixing chamber for mixing the prepared, conditioned carriermedium and the prepared pharmaceutical formulation to produce aconditioned aerosol, and

[0123] at least one measuring cell into which the conditioned aerosol isintroduced in order to carry out a measurement of the particle sizedistribution of the aerosol.

[0124] In advantageous embodiments of the apparatus, the or eachmeasuring cell for carrying out the impactor method is an impactor,preferably an Anderson cascade impactor.

[0125] In advantageous embodiments of the apparatus, the or eachmeasuring cell for carrying out the laser diffraction process has anleast one entry window for the entry of a light beam from a light sourceand at least one exit window for the exit of the scattered light fromthe light beam, while preferably the or each entry window is arranged ata tilted angle to the incident light beams from the light source andpreferably the or each exit window is arranged at a tilted angle to theincident light beams. Reflections are avoided by the fact that the lightbeam does not strike perpendicularly on the entry or exit window.

[0126] According to advantageous embodiments of the apparatus, the oreach entry window and the or each exit window are arranged to be tiltedat identical—but opposite—angles relative to the incident light beams.As a result of the two windows being tilted opposite ways and at thesame angle the offset of the incident and emergent light beams iscancelled out again because of the tilt.

[0127] In favourable embodiments of the invention, the or each entrywindow and/or the or each exit window are constructed to be removable.This makes it possible to clean not only the measuring cell but also thewindow itself after the measurement has been done and to investigate anydeposit of aerosol particles in the measuring cell and on the windows.

[0128] According to advantageous embodiments of the invention, the oreach entry window and the or each exit window are thin in construction,particularly less than 2 mm thick. An advantage of this embodiment isthat the incident or emergent light beam which preferably fallsdiagonally on the tilted windows and which is diffracted as it entersthe glass from the ambient atmosphere undergoes only a smalldisplacement of the beam if the windows are thin.

[0129] According to advantageous embodiments of the invention, the oreach measuring cell for carrying out the scattered light method has atleast one entry window for the entry of a light beam from a light sourceand at least one exit window for the exit of the scattered light fromthe light beam, the two windows preferably being arranged substantiallyat right angles to each other.

[0130] The problem of the equipment according to the invention is solvedby an apparatus, particularly an apparatus for carrying out the secondprocess according to the invention, which specifically has the followingcomponents:

[0131] a metering device for metering and preparing a pharmaceuticalformulation,

[0132] a mixing chamber for mixing a prepared carrier medium and theprepared pharmaceutical formulation to produce an aerosol, and

[0133] at least one measuring cell into which the aerosol is introducedin order to measure the particle size distribution of the aerosol by thelaser diffraction process and/or the scattered light method, the or eachmeasuring cell being integrated in the USP throat and forming, togetherwith this throat, an angled measuring cell.

[0134] The invention also discloses an angled measuring cell,particularly as a component or replacement part for an apparatus forcarrying out the process according to the invention, which comprises theUSP throat and at least one measuring cell integrated in the USP throat.

[0135] According to advantageous embodiments of the measuring cell, theor each measuring cell for carrying out the laser diffraction processhas at least one entry window for the entry of a light beam from a lightsource and at least one exit window for the exit of the scattered lightfrom the light beam.

[0136] According to favourable embodiments of the angled measuring cell,the or each entry window is arranged at a tilted angle to the incidentlight beams from the light source and preferably the or each exit windowis arranged at a tilted angle to the incident light beams, while the oreach entry window and the or each exit window are arranged to be tiltedat identical—but opposite—angles relative to the incident light beams.

[0137] In favourable embodiments of the angle measuring cell, the oreach entry window and/or the or each exit window are constructed to beremovable.

[0138] According to advantageous embodiments of the angled measuringcell, the or each entry window and the or each exit window are thin inconstruction, particularly less than 2 mm thick, the or each entrywindow and the or each exit window being arranged opposite each other.

[0139] According to advantageous embodiments of the angled measuringcell, the or each entry window and the or each exit window of the oreach measuring cell for carrying out the scattered light method arearranged substantially at right angles to each other.

[0140] The present invention is hereinafter described by reference to anembodiment by way of example illustrated in FIGS. 6a, 6 b and 7.Specifically, as noted previously,

[0141]FIG. 1: shows an Andersen cascade impactor with the USP throat inside view, partly in section,

[0142]FIGS. 2a to 2 d: show the USP throat with its standardisedgeometric measurements,

[0143]FIG. 3: shows an impactor nozzle shown diagrammatically and insection in side view,

[0144]FIG. 4: shows a diagram relating to the dependency of the dropletsize on the life of the droplet, for three different degrees ofsaturation with water,

[0145]FIG. 5: shows a diagram comparing the properties of a conventionalUSP throat with those of the angled measuring cell,

[0146]FIGS. 6a to 6 b: show an embodiment of an angled measuring cell indifferent side views, partly in section, and

[0147]FIG. 7: shows an embodiment of the apparatus for measuring theparticle size distribution of the aerosol.

[0148] In FIGS. 1, 2a-2 d, 3, 6 a-6 b and 7, the following numbers areused for the following parts:

[0149]1 Andersen cascade impactor

[0150]2 throat

[0151]2 a first section

[0152]2 b second section

[0153]3 entry opening

[0154]4 connecting member

[0155]5 sample collector

[0156]6 ₁ cascade

[0157]6 ₂ cascade

[0158]6 ₃ cascade

[0159]6 ₄ cascade

[0160]6 ₅ cascade

[0161]6 ₆ cascade

[0162]6 ₇ cascade

[0163]6 ₈ cascade

[0164]6 ₉ cascade

[0165]7 ₁ impactor nozzles

[0166]7 ₂ impactor nozzles

[0167]7 ₃ impactor nozzles

[0168]7 ₄ impactor nozzles

[0169]7 ₅ impactor nozzles

[0170]7 ₆ impactor nozzles

[0171]7 ₇ impactor nozzles

[0172]7 ₈ impactor nozzles

[0173]8 entry opening

[0174]9 ₁ exit opening

[0175]9 ₂ exit opening

[0176]10 ₁ flow lines

[0177]10 ₂ flow lines

[0178]11 impactor plate

[0179]12 flight path of a striking particle

[0180]20 angled measuring cell

[0181]21 entry window

[0182]22 exit window

[0183]23 measuring cell

[0184]24 conditioning device

[0185]25 mixing chamber

[0186]26 metering device, nebuliser

[0187]27 laser

[0188]28 lens

[0189]29 semiconductor detector

[0190]FIG. 6a shows a first embodiment of an angled measuring cell 20which comprises a USP throat 2 and a measuring cell 23 for carrying outthe laser diffraction process. The measuring cell is arranged in thefirst section 2 a of the USP throat, so that the current of aerosol ismeasured even before the first particle fraction has been deposited inthe angled section at the transition from the first section 2 a to thesecond section 2 b.

[0191] The measuring cell 23, shown in section, has an entry window 21and an exit window 22. The two windows 21, 22 are constructed to beremovable so that after the measurement has been done the measuring cell23 can be cleaned and examined. Both the entry window 21 and the exitwindow 22 are tilted slightly, relative to the incident light beam, thuspreventing reflection of the incident light beam. The two windows 21, 22are tilted at the same angle, but opposite one another, in order tocompensate for the offset of the beam of the incident laser light as aresult of the tilt.

[0192] It is clearly apparent that the entry window 21 is relativelysmall compared with the exit window 22. The reason for this is that theincident light coming through the entry window 21 is an undisturbed beamof light which is propagated in linear manner, whereas the light leavingthrough the exit window 22 is the scattered light from the incidentlight beam diffracted on the aerosol particles, for which reason theexit window 22 should also be constructed so that scattered light from alarge enough angular area can be accommodated, i.e. is able to leave themeasuring cell 23 and be picked up by a detector.

[0193]FIG. 6b shows the angled measuring cell 20 shown in FIG. 6a, in aview turned through 90° relative to the position shown in FIG. 6a,partly in section.

[0194] It clearly shows the USP throat 2 consisting of the first section2 a and the second section 2 b, and the measuring cell 23 integratedtherein in the first section 2 a. In this way the aerosol flow enteringthrough the inlet opening 3 is measured before it is deflected throughthe angled section into the second section 2 b. The Figure also showsthe flight path of a particle striking the inner wall of the secondsection 2 b which with the other particles arriving at this point formsthe first deposited fraction of the aerosol flow.

[0195]FIG. 7 shows an embodiment of an apparatus for measuring theparticle size distribution of an aerosol, shown diagrammatically.

[0196] The apparatus comprises a conditioning device 24 for saturating acarrier medium with a conditioning agent to a given degree ofsaturation. The carrier medium thus conditioned is fed into a mixingchamber 25 which contains a nebuliser to be examined, which atomises asolution of pharmaceutical substance stored therein. The nebuliser 26acts as a metering device 26 for metering and preparing a pharmaceuticalsolution. The pharmaceutical solution prepared using the metering deviceis mixed with the conditioned carrier medium in the mixing chamber 25 toform a conditioned aerosol.

[0197] The aerosol conditioned in this way is introduced into ameasuring cell 23.

[0198] This measuring cell 23 is arranged in a first section 2 a of theUSP throat 2 and together with the throat 2 forms an angled measuringcell 20, which is surrounded by a dotted line in FIG. 7.

[0199] The light emitted by the laser 27 falls through the entry window21 into the interior of the measuring cell 23 and from there strikes theaerosol flow guided by the first section 2 a of the USP throat 2. Theincident laser light is diffracted on the particles of the aerosol,which constitutes an obstacle to the light. The scattered light which isgenerated by the diffraction of the incident laser light on theparticles of the aerosol leaves the measuring cell 23 through the exitwindow 22, is then focussed through a lens 28 and fed into asemiconductor detector 29 for evaluation. After being measured in themeasuring cell the aerosol flow is diverted into the second section 2 bof the USP throat 2. After passing through the throat 2 or the angledmeasuring cell 20 the aerosol flow measured by the laser diffractionprocess can be fed into a particle separator in order to obtainquantitative information on the particle mass; the particle separatormay take the form of a filter.

[0200] An additional measuring cell for obtaining a measurement by thescattered light method could be provided, in which case this measuringcell would be arranged both in the first section 2 a and in the secondsection 2 b of the USP throat 2. This would also make it possible toobtain quantitative information on the particle mass.

[0201] The apparatus shown in FIG. 7 could also have an Andersen cascadeimpactor adjacent to it, which could be used inter alia to verify theparticle size distribution of the aerosol determined by the laserdiffraction method.

[0202] On the basis of the foregoing, the present invention can besummarised as follows:

[0203] The measurement of the particle size distribution of an aerosolis of crucial importance in the pharmaceutical industry. Theprerequisite for this is a valid method, e.g. the cascade impactormethod described in the pharmacopoeias. It requires measurement of theactive substance concentration in order to determine the particle sizedistribution.

[0204] However, the above method and other methods as well rapidly leadto an increase or decrease in the pH, which may lead to decomposition ofsensitive active substances. Consequently, the particle sizes can nolonger be determined precisely, as it is usual to determine theconcentration of active substance in order to obtain the particle size.

[0205] Surprisingly, it has been shown that by adjusting the relativehumidity to 80 to 100%, preferably 90 to 100%, using the solvent onwhich the solution or suspension formulation is based, preferably wateror a water/alcohol mixture (ethanol), the breakdown of the activesubstance during the measuring process can be totally prevented.

[0206] Another advantage of the process according to the invention isthat the particle size can be determined irrespective of the nature ofthe aqueous formulation, i.e. different salt concentrations in thesolution or suspension have no effect on the measurement of the particlesize.

[0207] It has also advantageously been found that the measurement of theparticles is not disrupted when water condenses out in the measuringapparatus, particularly the cascade impactor, during the measurement.

[0208] Compared with the standard measurement without conditioning theair with a solvent such as water the relative standard deviation can beimproved from 10-15% to about 2% using the process according to theinvention.

[0209] At a relative humidity of 100% the method shows a significantlylower scattering in the MMAD (mass median aerodynamic diameter) than thestandard method without conditioning of the air with a solvent such aswater.

[0210] For the process according to the invention, water saturated withmoisture can be produced by passing compressed air through a water bathheated to above ambient temperature; the water temperature is preferablyabout 30 to 45° C.

[0211] The process is preferably used to measure the spray pattern of apropellant-free “soft mist inhaler”, preferably known under thetrademark Respimat®, disclosed for example in WO 97/12687. Preferredformulations are described in WO 97/01329 and WO 98/27959, to whichreference is hereby expressly made.

What is claimed is:
 1. In a process for determining the sizedistribution of the particles contained in an aerosol of apharmaceutical formulation, the improvement which comprises: preparing acarrier medium which is saturated with a conditioning agent according toa given degree of saturation, mixing the conditioned carrier medium witha pharmaceutical formulation to produce a conditioned aerosol,introducing the aerosol into at least one measuring cell; and measuringthe conditioned aerosol in the measuring cell to determine the particlesize distribution of the aerosol.
 2. The process according to claim 1,characterised in that the conditioned aerosol is measured by the laserdiffraction method.
 3. The process according to claim 1, characterisedin that the conditioned aerosol is measured by the scattered lightmethod.
 4. The process according to claim 1, characterised in that theconditioned aerosol is measured by the impactor method.
 5. The processaccording to claim 1, characterised in that the conditioned aerosol ismeasured by the laser diffraction method and the scattered light method.6. The process according to claim 5, characterised in that theconditioned aerosol is additionally measured by the impactor method. 7.The process according to claim 1, characterised in that thepharmaceutical formulation is a solution of pharmaceutical substance ina solvent, and the solvent in which the pharmaceutical substance isdissolved is used as the conditioning agent.
 8. The process according toclaim 1, characterised in that the pharmaceutical formulation is asuspension of pharmaceutical substance in a suspension agent, and thesuspension agent in which the pharmaceutical substance is present in theform of suspended particles is used as the conditioning agent.
 9. Theprocess according to claim 1, characterised in that in the case of apowdered pharmaceutical formulation where a pharmaceutical substance isin powdered form, water is used as the conditioning agent.
 10. Theprocess according to claim 1, characterised in that air is used as thecarrier medium.
 11. The process according to claim 1, characterised inthat the measurement of the particle size distribution of the aerosol iscarried out in at least one measuring cell separated from thesurrounding atmosphere.
 12. An apparatus for determining the sizedistribution of particles contained in an aerosol of a pharmaceuticalformulation, comprising: a conditioning device (24) for saturating acarrier medium with a conditioning agent according to a given saturationlevel, a metering device (26) for metering a pharmaceutical formulation,a mixing chamber (25) for mixing the conditioned carrier medium and themetered pharmaceutical formulation to produce a conditioned aerosol ofpharmaceutical formulation, and at least one measuring cell (23) intowhich the conditioned aerosol of pharmaceutical formulation isintroduced in order to carry out a measurement of the particle sizedistribution of the aerosol.
 13. The apparatus according to claim 12,characterised in the measuring cell is an impactor.
 14. The apparatusaccording to claim 13, characterised in that the impactor is an Andersoncascade impactor.
 15. The apparatus according to claim 12 characterisedin that the measuring cell has at least one entry window (21) for theentry of a light beam from a light source and at least one exit window(22) for the exit of the scattered light from the light beam.
 16. Theapparatus according to claim 15, characterised in that the entry window(21) is tilted relative to the incident light beam from the lightsource.
 17. The apparatus according to claim 15, characterised in thatthe exit window (22) is tilted relative to the emergent light beam. 18.The apparatus according to claim 15, characterised in that the entrywindow (21) and the exit window (22) are tilted at identical—butopposite—angles relative to the incident light beam.
 19. The apparatusaccording to claim 15, characterised in that the entry window (21) andthe exit window (22) are constructed to be removable.
 20. The apparatusaccording to claim 15, characterised in that the entry window (21) andthe exit window (22) are less than about 2 mm thick.
 21. The apparatusaccording to claim 15, characterised in that the entry window (21) andthe exit window (22) are arranged opposite one another.
 22. Theapparatus according to claim 15, characterised in that the exit windowand the entry window of the measuring cell are arranged substantially atright angles to one another.